Cleaving thin waters from crystals

ABSTRACT

A method of creating thin wafers of single crystal silicon, sapphire and similar materials, wherein an ingot of single crystalline material, or a ribbon of single crystalline material is cleaved, in a plane parallel to a surface, with laser light focused to a line in the desired plane of cleavage, near the growing cleavage furrow. The light is of a wavelength that the material is transparent to, but for which the material has strong two- or three-photon absorption. Consequently the light is not appreciably absorbed until it reached the desired focal line. The light is presented in an extremely short pulse, which heats and expands the material at the line focus, before the heat can be dissipated. This expansion creates tangential stresses around the focal line. These stresses are designed to be precisely normal to the growing cleavage furrow. Therefore the stresses are able to induce cleavage in the desired plane, without inducing cleavage in other possible cleavage planes that may happen to intersect with the growing cleavage edge. In this way, extremely thin wafers and ribbon shaped wafers can be produced, with extremely high quality cleaved faces. Methods of initiating the cleavage furrow and separating the cleaved wafer from the rest of the crystal are also discussed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/761,547 filed Jan. 24, 2006, from U.S.Provisional Patent Application Ser. No. 60/745,035 filed Apr. 18, 2006,from U.S. Provisional Patent Application Ser. No. 60/745,759 filed Apr.27, 2006, from U.S. Provisional Patent Application Ser. No. 60/808,553filed Jul. 5, 2006, and from U.S. patent application Ser. No. 12/162,033filed Jan. 24, 2008, the entire disclosures of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the technology of making wafers fromcrystalline materials such as silicon or sapphire, and in particular tothe problem of cleaving thin wafers, including wafers elongated intoribbons, from single-crystals of such materials.

DESCRIPTION OF THE PRIOR ART

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 12/162,033, which taught the use of preciselyfocused lasers of carefully chosen parameters to cleave wafers fromsingle crystals. That application focused on crystals of silicon, butthe application made it clear that the method was not limited tosilicon, and could be applied to cleaving thin wafers from crystals of awide variety of materials, including those sometimes thought to bedifficult or impossible to cleave. The necessary property of thematerial was that there was a wavelength at which the material wastransparent, but where an ultrafast pulse of light at such wavelengthwas focused into the material to a focal region, there could beabsorption at the focal region by multiphoton absorption. The presentcontinuation-in-parts still discusses silicon, but emphasizes thatgenerality by discussing application of the method to crystals ofsapphire in addition to those of silicon.

Most photovoltaic solar cells produced today are based on crystallinesilicon, either single-crystal silicon or polycrystalline silicon. Thesingle-crystal silicon cells convert solar energy more efficiently, butare more expensive. Because the cost of the silicon substrate iscurrently a major component in the cost of these cells, any improvementthat enabled practical and low cost production of thinner single-crystalsilicon wafers could significantly lower the cost of the cells, andenable the production of a cell that was both efficient and economical.

Two methods for producing wafers from ingots of single-crystal siliconare 1) sawing them, followed by removal of the saw-damaged superficiallayers and polishing and 2) creating a subsurface layer of weakenedsilicon by means of ion implantation, generally by hydrogen ion beams,followed by attachment of a stiffening layer to the top surface andremoval of the stiffening layer and the top surface as a unit from thecrystal (Bruel, U.S. Pat. No. 5,374,564). The sawing method results inmuch wasted silicon, and this wasted silicon adds a significant cost tosolar cells produced from the wafers. The sawing also produces wastesthat must be dealt with through recycling or disposal, adding additionaleconomic and environmental cost to the cells. Furthermore there is addedexpense from removal of the damaged surface, polishing and cleaning theindividual wafers before further processing can take place.

The ion beam deposited layer method is practically limited to wafersthat are less than 10 microns in thickness because of the energyrequirements of the penetrating ion beam. Furthermore, it requires arelatively long cycle time between the production of successive wafersfrom the ingot. It requires cycling between different vacuum andtemperature environments. The ion beam can transfer contaminants to thewafer. Finally, because of crystal surface damage left after separation,the newly produced wafer requires removal of the superficial layer andpolishing. While this ion beam process is currently used in theproduction of silicon-on-insulator wafers, where the thinness of theresulting silicon layers offers performance advantages, and where therelatively high costs per wafer can be absorbed in the value added tothe wafer by integrated circuit manufacture, it would be prohibitivelyexpensive for silicon solar cell production.

In theory, cleavage of a thin layer from the end of a single-crystalingot of silicon could produce wafers with intrinsically perfectsurfaces, requiring no further polishing, and with no loss of materialdue to sawing. However in view of the brittleness of silicon, notechnique that avoids the need for creation of a weakened layer by ionimplantation has yet proven to be a practical method for cleaving wafersfrom single-crystal silicon with both the thinness and the areaeconomical for solar cells.

Currently sapphire wafers are made by three different methods, 1) sawingwafers from boules of single crystal sapphire, 2) drawing ribbons from amelt, and 3) vapor depositing a layer of sapphire on a substrate andthen freeing the newly formed sapphire layer from the substrate. Thefirst of these methods is costly, in view of the hardness of sapphire,and the needed smoothing and polishing operations following sawing. Themethod also involves loss of materials due to the kerf in the sawingprocess. Furthermore this method is intrinsically incapable of producinglong ribbons of sapphire that could be used in roll to roll processes.The second of these methods can produce long ribbons, but there arelimitations how thin the sapphire can be. And the third of these methodsrequires costly processing techniques. So there is an unmet need for aprocess capable of economically producing very thin ribbons of sapphirewith high quality surfaces.

Objects and Advantages

One object of the present invention is to provide an economical methodof production of wafers and ribbons of silicon, sapphire and othercrystalline materials.

Another object is to provide a silicon wafer production method thatinvolves no wasted material due to saw kerf.

Another object is to provide such a method that produces wafers that areintrinsically smooth enough to require little or no additionalpolishing.

Another object is to provide such a method where wafers are produces ina state that requires no further cleaning before additional processing.

Another object of the present invention is to provide such a cleavagemethod that can operate at room temperature.

Another object is to provide stresses to cleave the silicon, sapphireand similar materials that are greatest at the growing edge of thecleavage, and are low other places, particularly at the vulnerable outersurfaces of the crystal.

Another object is to provide such a method that provides a rapid cycletime between production of successive wafers from the same ingot.

Another object is provide a method of making wafers and ribbons as thinas 25 microns, or even thinner, without requiring additional smoothingand polishing.

Another object is to provide wafer production by a method thatintroduces substantially no impurities into the wafer or ribbon.

Another object is to provide a technique for wafer production wherewafer area is limited only by the cross sectional area of availableingots.

Another object is to reduce the waste products in wafer production,thereby reducing the environmental and economic cost of the finishedsolar cell.

Another object of the invention is to economically produce crystallineribbons wherein two ribbons of different crystalline materials arelaminated to produce a laminate having electrical, optical and/ormechanical properties superior to those of either of the individualmaterials.

SUMMARY OF THE INVENTION

An ingot of single-crystal silicon with a (111) axis is flattened andpolished at one end normal to the axis, and a notch with a vertex in the(111) plane is produced on a side of the ingot, such that the distancebetween this vertex and said end is the desired thickness of a wafer tobe cleaved from the ingot and such this vertex is in the desired planeof cleavage. Light of a wavelength able to penetrate into the siliconcrystal without significant absorption, when the intensity of the beamis low, but is efficiently absorbed and converted to heat when theintensity of the beam is high, is focused to an elongated volume with anaxis of elongation in the desired cleavage plane, parallel to and ashort distance from said notch edge. Heating and the resulting transientlocal expansion of the silicon in this illuminated volume causes tensilestress at the vertex of said notch, substantially normal to the desiredcleavage plane, thereby causing fracture of the crystal in the chosencleavage plane. Movement of the illuminated volume relative to the ingotallows the fracture to propagate across the desired cleavage plane,thereby completely severing the wafer from the rest of the ingot. Thissame process can also be applied to cleaving a crystalline ribbon ofsilicon, sapphire or some other crystalline material, produced byvarious processes in a plane parallel to the face of the ribbon, thusgenerating thinner ribbons.

SUMMARY OF THE DRAWINGS

FIG. 1 shows a three-dimensional view of the silicon ingot showing itscrystal axis and the plane of the desired cleavage.

FIG. 2 is a schematic plan view of the invention showing the means tolocally heat a volume of the ingot in a region of the plane of thedesired cleavage.

FIG. 3 is a three-dimensional view of the device shown in FIG. 2,adapted for a rectangular ingot with a notch in one side.

FIG. 4 is a detail of FIG. 3, showing the full width of the illuminatedvolume in relationship to the starting notch.

FIG. 5 is schematic cross sectional view of the elements shown in FIG. 4showing stresses in the ingot produced by local heating in relationshipto the starting notch.

FIG. 6 is a three-dimensional view of the device shown in FIG. 2 adaptedfor a rectangular ingot with a notch in one edge.

FIG. 7 is a three-dimensional view of the device shown in FIG. 2,adapted for a producing a notch in an edge of a rectangular ingot at thetime of cleavage.

FIG. 8 is a three-dimensional view showing a system for sequentiallydirecting the heating laser to successive segments along the heatedvolume.

FIG. 9 shows a technique for creating a focused line of light, but wherenear the focus, the intensity effectively falls off inversely with thesquare of the distance from the focus.

FIG. 10 is a three-dimensional view showing apparatus for producing aset of short parallel cleavage fractures normal to one edge of arectangular ingot.

FIG. 11 is a three-dimensional view showing an ingot after processing bythe apparatus illustrated in FIG. 10.

FIGS. 12A, 12B and 12C are schematic cross sectional views showing threesuccessive time points in a process for peeling a cleaved wafer from thebalance the ingot from which it was cleaved.

FIGS. 13A, 13B and 13C are three-dimensional views showing how a newlycleaved wafer can be mounted in a supporting frame for furtherprocessing.

FIG. 14 is a schematic cross sectional view showing the relationshipbetween the wafer and the supporting frame.

FIG. 15 is a three-dimensional view showing how wafers supported inframes can be carried in super-frames, linked together to pass through acell processing line.

FIG. 16 is a schematic cross sectional view showing how a prospectiveplane for cleaving can be optically examined for inclusions beforebeginning of the cleavage operation.

FIG. 17 is a schematic cross sectional view showing apparatus forincreasing the sensitivity of detection of inclusions compared to theapparatus shown in FIG. 16.

FIG. 18 is a schematic cross sectional view showing how a ribbon waferof silicon, sapphire or a similar material can be cleaved in planesparallel to the face of the ribbon, to create three ribbons of one thirdof the thickness of the original ribbon.

FIG. 19 is a schematic cross sectional view showing how ribbons, perhapsof different compositions and/or crystal orientations, and pulled by ahorizontal float process from two different crucibles, can each becleaved in half, to create two ribbons of the same width but half thethickness of the original ribbon, and then the newly formed ribbonslaminated to form new ribbons, each combining ribbons from the differentcrucibles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 shows an ingot 1 of single-crystal silicon. To simplify thepresent discussion, ingot 1 has been ground to a rectangular prismshape. One face 2 of the ingot, normal to the main crystal axis(identified by an arrow), is ground and polished, and will serve as oneface of the first wafer to be cleaved from the ingot by the method ofthe present invention. The desired cleavage plane is parallel to face 2and is shown by the dotted line 3 where it intersects a face 4 of ingot1 perpendicular to face 2. On another face 5 of ingot 1, perpendicularto both faces 2 and 4, is a notch 6 with a vertex in the desiredcleavage plane. Since the (111) plane is the cleavage plane of silicon,the main axis of ingot 1 is oriented in the (111) direction. Inpractice, in order to maximize utilization of the full cross sectionalarea of the silicon ingot, the ingot might be left in its existingcylindrical shape, with notch 6 produced on one side. Furthermore, theideal location of the notch is on the edge formed by two intersectingplanes ground on the sides of an ingot, for example to form arectangular ingot.

To promote cleavage, notch 6 should terminate in as sharp a vertex aspossible. One way to create a notch with a sharp vertex is to usephotolithography to create a narrow slot in a photoresist layer on face5 so that a narrow strip of silicon can be exposed to a chemical etchingsolution, such as KOH, which at 80° C., etches (100) planes 400 timesmore rapidly than (111) planes resulting in sharp edges at theintersections of two (111) planes (Suwito et al., J. Appl. Physics83:3574 (98)). To use that etching procedure, the notch should be at theintersection of two (111) planes, one of which is the desired cleavageplane, and the other of which is oriented at 70.5° with respect to thatplane. The resulting notch will be a sawtooth. The notches might also bemade by any other method that may produce sharp notches in silicon, suchmilled in the silicon by a focused ion beam, produced by a focusedpulsed uv laser, or produced by an extremely sharp diamond blade. In thecase of a notch made with an extremely sharp diamond blade, the notchcould be made at the time of the laser pulse, by contacting the ingotwith the blade in substantial synchrony with the pulse, so as to have asynergistic effect in initiating the cleavage, as will be describedbelow in more detail.

Considering a cylindrical ingot, KOH etching will only create sawtoothshaped notches at regions where the line of intersection of two (111)planes is tangent to the surface, and these only occur at intervals of60° around the periphery. Cleavage initiated at such regions has aproblem because cleavage might be initiated at the wrong (111) planepassing through the growing fracture edge. This problem is addressed inthe present method, since the wrong plane is generally under compressivestress while the correct plane is under tensile stress. During thepeeling process, discussed later, this problem might be addressed byinitiating peeling from a point 30° offset from the notch region, tominimize the chance of unwanted fracture. A notch made initially with adiamond blade, on the other hand, might be optimally made in at thispoint 30° offset from the two points around the ingot when theintersection of two (111) planes is tangent to the ingot surface.

FIG. 2 shows how a laser 10 is focused by cylindrical lens 11 to avolume in the interior of ingot 1, such that the volume is veryelongated along an axis that is in the desired cleavage plane and isparallel to and a short distance from the sharp vertex of notch 6. Ananamorphic beam expander 12 expands the output of laser 10 into acollimated beam with a long rectangular cross section to match the areaof cylindrical lens 11. Though lens 11 is shown as a single element, itcould be a compound lens. Lens 11 could also be circularly symmetrical,and adapted to focus a rapidly scanned point onto a line, and stillother possibilities are discussed below.

FIG. 3 shows in the device of FIG. 2 in three dimensions, and shows theilluminated volume 13 in the ingot. The illuminated volume 13 isapproximately a cylinder with its axis in the desired cleavage plane andparallel to the vertex of notch 6. Generally, the cylinder is distortedto have an oval cross section with its width in the desired cleavageplane greater than in dimension parallel to the optical axis of lens 11.When the width of the illuminated volume is made as narrow as possible,given the limitations of the optics, this oval cross section resultsfrom the fact that the three dimensional point spread function has agreater width in the longitudinal than in the lateral dimension. In anycase, the shape and position of the illuminated (and heated) volume 13is chosen so that at the edge of the notch 6 or at the growing edge ofthe cleavage fracture extending from that notch edge, the stressesnormal to the desired cleavage plane are tensile and they are maximal atthe desired cleavage growth front.

FIG. 4 is a detail from FIG. 3 showing the full length of illuminatedvolume 13. It should be understood that in FIG. 3 and the otherdrawings, the size of the notch 6 and the heated volume 13 are shownmuch larger in relationship to other illustrated elements than theiractual size, since in most cases the diameter of volume 13 in the planeof cleavage would be no more than a few microns and would be just a fewmicrons away from the vertex of notch 6, while the ingot width could be10 cm or more.

A pulse of light from laser 10 heats the silicon in the illuminatedvolume 13 very rapidly, before there has been time for significantconduction of the heat away from the volume. In FIG. 5 the illuminatedvolume 13 is shown in cross section. On heating, the silicon in thevolume 13 expands, creating radial compressive stress in the surroundingcooler silicon, shown by the dotted arrows in FIG. 5, and tangentialtensile stress, shown by the curved solid arrows in FIG. 5. The vertexof notch 6 is subject to this tensile stress, normal to desired cleavageplane, and because of the notch, the stress is maximal near the vertex.The degree of heating is chosen so that the maximal stress near thevertex is just above the threshold for initiation of cleavage. Thereforea fracture is initiated at the vertex of notch 6 in the desired plane,but nowhere else. The fracture only can propagate a short distance,because the tensile stress gets smaller closer to the axis of theilluminated volume. After the initial heating caused by the laser pulse,the temperature in the silicon is allowed to equilibrate. The ingot andthe focusing system are moved relative to each other so that theilluminated volume is a small distance further from the notch 6, butwith its axis remaining in the desired cleavage plane, and a new laserpulse of light repeats the process to extend the fracture.

This process is repeated several more times until the length of thefracture extending from the notch is great enough so that theilluminated volume can be positioned between the notch and the far edgeof the fracture. Now a laser pulse can still create tensile stress onthe growing fracture, but the fracture propagates away from theilluminated volume rather than towards it, so there is no need forequilibration of temperature between successive pulses, and the cylindercan be moved steadily away from the notch, keeping a constant distanceto the growing fracture end, until the far end of the ingot is reached,and the cleavage has extended over the entire area of the ingot, tocompletely sever the wafer. Although the growing edge of the cleavagefracture might be visualized by ultrasonic microscopy, for example, inpractice there may be no need for such visualization. The optimumadvance of the illuminated volume relative to the ingot on successivecould be determined empirically, and such scanning would bepreprogrammed, and implemented by a computer-controlled translator.

To produce defect-free cleaved surfaces with this process, it iscritical to maintain a substantially pure tensile stress normal to thecleavage plane near the fracture end, and to stay below the thresholdfor new fracture initiation. Centering the heated volume on the cleavageplane to maintain symmetry with that plane facilitates this. To createsuch symmetry it is important that there be no significant absorption ofthe light traveling to the volume, but that the absorption be veryefficient at the volume. These seemingly incompatible requirements aremet by light in the wavelength range from around 1.2 to 3 μm, presentedin pulses less than a few ns in duration in the short wavelength part ofthis range and less than a few hundred fs in the longer wavelengthregion of this range. This is because silicon is relatively transparentin this wavelength range, and only when the intensity becomes muchgreater, with short pulses and in the region of the beam focus, is theintensity great enough for two-photon or three-photon absorption, whichdepend on the square or the cube of the intensity, respectively.Therefore the light of lower intensity converging on the line focuswould pass with relatively low absorption through the crystal, and onlyat the focus would the light be absorbed and converted into heat. (Xuand Denk, J. Appl. Phys 86:2226(99), Nejadmalayeri, et al., Opt. Lett30:964 (05)).

In the short wavelength end of the 1.2 to 3 μm range, two-photonabsorption predominates, and for wavelengths longer than about 2.2 μm,only three-photon absorption is possible. Each of these multiphotonabsorption processes has certain advantages and disadvantages.Two-photon absorption does not require as high peak intensities, and cantake advantage of the greater range of commercially available powerfulshort-pulsed lasers in the wavelength range between 1.2 to 2.2 μm.However, when the laser light is focused to a line, the intensity fallsoff inversely with just the first power of distance from the line focus,and combined with the quadratic dependence of two-photon absorption, theresult does not provide very large discrimination between the focalplane and planes close to that plane. This problem is exacerbatedbecause the high pulse powers needed for sufficient heating can lead tointensities able to trigger two-photon absorption at relatively largedistances from the focal plane.

Three-photon absorption, between about 2.2 to 3 μm, places greaterdemands on the brevity of the laser pulses, which in combination withthe pulse energy requirements severely limits the choice of availablelasers. However three-photon absorption provides much better depthdiscrimination, and in particular it should work well even with thesimple device shown in FIG. 3.

One commercially available laser source that should be suitable forthree-photon absorption induced heating with the apparatus shown in FIG.3 is the Spectra-Physics OPA-800C optical parametric amplifier pumped bya Spitfire amplified Ti-sapphire laser (Spectra-Physics, Mountain View,Calif.). This combination, referred to here simply as a “laser,” couldbe tuned to 2.4 μm, as in the study of Nejadmalayeri, et al., Opt. Lett30:964 (05). That study showed that the output pulses of that laser,focused to a point in the interior of a volume of silicon, aresufficient to raise the temperature at the focus to the high levelsneeded to produce a lasting refractive index change. In the presentinvention, when the beam is focused to a line segment, because the focalvolume includes a larger mass of silicon, lower temperatures would beproduced. Such lower temperatures hopefully would not produce anylasting change in physical properties, but would be sufficient toproduce the local stresses sufficient to initiate and extend a cleavagefracture, as described in this specification. That laser also has awidely adjustable wavelength output, so the initial set-up could be usedto collect data to help choose the best wavelength and other parametersto optimize the process for a production environment. With theconfiguration of FIG. 3, the output pulse energy of theOPA-800C/Spitfire laser at 2.4 μm should be sufficient to cleave atleast narrow wafers.

An alternative, and potentially much more economical, source of theultrashort pulses at the desired mid-IR wavelength are newly developedfiber based lasers (Sorokina et al IEEE Journal of Selected Topics inQuantum Electronics, 20 (5) 2014),

With notch formation by methods such as chemical etching or ion millingdiscussed above, the vertex of the notch 6 would have a substantiallylarger radius of curvature than the atomic dimensions of the growingedge of a cleavage fracture, so higher stresses would be required toinitiate cleavage at the notch vertex than to extend the growingcleavage edge. Consequently, lower maximum temperatures are required atilluminated volume 13 for cleavage extension than for cleavageinitiation. This difference suggests a modification of the apparatus ofFIG. 3, where the notch is on an intersection edge of a two flat areasground into the side of an ingot, so that only a small length of theheated volume is required at the site of cleavage initiation. Such anarrangement is shown in FIG. 6, which is the same as the device shown inFIG. 3 except for the location of the notch at an edge, and the factthat anamorphic beam expander 12 (unillustrated) can be optionally movedout of the path of the laser beam. For cleavage initiation, theanamorphic beam expander 12 is moved out of the beam path, so the muchnarrower collimated output beam from laser 10 (unillustrated) isdirectly incident on lens 11, and consequently all the pulse energy isconcentrated on a much shorter illuminated volume 13, of a length, say,on the order of a millimeter or less. (If necessary, even moreconcentrated illumination is possible by replacing the beam expanderwith a beam area reducer.)

After the cleavage fracture is started, the beam expander 12 is put backin the beam path, for the scan to the opposite edge of the ingot.Because the focused laser beam used for cleavage extension can be oflower peak power per unit length of the illuminated volume than for thedevice of FIG. 3, the illuminated volume 13 can be longer for a givenlaser pulse energy, and thus wider wafers can be produced than can beproduced by the device of FIG. 3.

The device of FIG. 7 shows a variant of the device of FIG. 6, where thenotch is made at the time of cleavage by an extremely sharp diamondblade 14, which is placed in contact with an edge 15 of ingot 1. Becausethe edge of the blade 14 can be aligned so it is in the plane of thefocal volume 13, there is no need for a step, required in the apparatusshown in FIG. 3 and FIG. 6, to insure that the vertex of notch 6 is inthe focal plane of illuminated volume 13. Additionally, there is no needto process the ingot by production of the notches such as notch 6 priorto the cleaving step. Finally, by applying a force to blade 14 while thesilicon it is contact with is under maximum tensile stress, thelikelihood of initiating cleavage at the intended location is greaterthan in the devices of FIG. 3 and FIG. 6, when the tensile stressessimply encounter a notch 6 that concentrates them into the region of thedesired cleavage initiation.

With such an embodiment of the present invention, it may be desirable tomake the edge 15 particularly free of microcracks that could initiateunwanted cleavage in respond to transient heating. Therefore it may beuseful to expose edge 15 to chemical etching and polishing to remove theouter silicon layers along with any surface damage due to cutting andgrinding, prior to beginning the cleaving procedure. It may also beuseful to apply a protective coating to the edge 15 to reduce thechances of new cracks forming during handling of the ingot.

The requirement of synchrony between the laser pulse and force on blade14 could be met by the transient local expansion of the silicon inresponse to the laser pulse, or it could be provided by a piezoelectricactuator translating blade 14 in response to an electric pulse timed tobe in synchrony with the laser pulse. As an alternative synchronizationmeans, the output of laser 10 might be divided into two paths, one ofwhich is focused on the illuminated volume 13 and the other of which isdirected onto a light-to-mechanical force transducing element, such as asmall vane that converts the momentum of the optical pulse into amechanical impulse, thereby transiently increasing the force between theblade 14 and the ingot. In case the static force between the blade andthe ingot prior to the light pulse is too low to produce any lastingdeformation, the first microcrack created in response to the light pulsewould become a starting notch, which would then seed further extensionof the cleavage. Because of its simplicity and because it can be quicklyconstructed, the embodiment of the present invention shown in FIG. 7 isthe preferred form of the present invention, at least initially.

For cleaving still wider wafers, the pulse energy output of theOPA-800C/Spitfire could be increased, for example by increasing thepower of the pumping laser. Alternatively, a more powerful laser couldbe substituted. Another alternative is to focus the laser output onto ashort line segment which would be scanned in two dimensions over thedesired cleavage plane. To avoid the mismatches of cleavage planes thatcan result when two cleavage fronts intersect, patterns for this twodimensional scan should always extend the cleavage from an existingcleaved region, should always require the minimum scan movement betweentwo successive light pulses and should avoid the possibility of onecleaved region extending into a preexisting cleaved region. One exampleof pattern meeting these criteria (thinking of an ingot that has beenmachined to a rectangular cross section) would be to form notch 6 in oneedge of the ingot, then extend the cleavage from that notch to form asmall triangular cleaved region at an edge of the ingot. Then startingfrom that edge, the illuminated line segment is scanned along one of theingot sides to produce a cleaved margin adjacent to that side, andfinally, beginning in that margin, starting from one side of the ingot,the scanning is in parallel rows, perpendicular to the side with thecleaved margin, like mowing a rectangular lawn in strips, until theentire ingot is cleaved. It should be appreciated that with such ascheme, the wafer diameter size is limited only by the diameter of theingot, and wafers could be produced with dimensions far greater thanwould be practical with wire sliced ingots. Ribbon based waferproduction methods intrinsically have the ability to produce largedimensions in the length dimension, but the present method would allowlarge dimensions in width as well. Such large wafers improve powerconversion efficiency by reducing edge effects, and they also reducemodule assembly costs.

The scanning time in such a two-dimensional scanning scheme could bereduced considerably if scanning in the direction parallel to thegrowing cleavage fracture edge did not require relative movement betweenthe lens 11 and the ingot surface. This would be possible if suchscanning were performed with a continuously rotating polygon mirror or agalvanometer-operated plane mirror scanner, as shown in the apparatus ofFIG. 8. Successive output pulses of the laser 10 (unillustrated) arefocused in succession on collinear points 16 through 23, by aone-dimensional mirror scanner (also unillustrated). The art of suchmirror scanning such that the scanned points are coplanar, is well know,for example for laser printers, and will not be described further here,except to say that the optics would have to be compatible with themid-infrared light. Light diverging from point 16 is directed to lens24, which collimates it, and directs the collimated beam to one of theend portions of cylindrical lens 11, which then focuses the light to aline segment, heating up a volume of the silicon shown by region 13. Thenext pulse is focused on point 17, and the light is collimated by lens25 and focused in the silicon ingot to heat a volume adjacent to region13, and so on until the last region along the illuminated volume isheated. By making the light leaving lenses such as 24 and 25 slightlydivergent, the heated volumes will slightly overlap at their ends.

As mentioned above, two-photon absorption of light from about 1.1 to 2.1μm has advantages over three-photon absorption in that it places lessstringent requirements on the brevity of the pulses, to insure adequateinstantaneous intensities. Furthermore, relatively inexpensive lasersare available in that wavelength range with sufficient power per pulseto allow rapid scanning for high throughput. One problem, however, isthat the two-photon absorption of silicon is great enough so that at theintensities needed to deliver sufficient energy to the focal plane toinduce cleavage, in a short enough time interval to prevent dissipationof the thermal gradients, there can be substantial two-photon absorptionoutside of the focal region. A second problem is that when light isfocused to a line, the fall off of intensity from the focal planefollows a 1/r law, as opposed to the 1/r² law for light focused to apoint. These two factors in combination can make it difficult to get theideal pattern of tensile stresses normal to the cleavage plane, at thecleavage front. A possible solution to the first problem is to choose awavelength near the long wavelength cutoff for two-photon absorption,where absorption is relatively inefficient compared to the wavelengthsof maximum absorption, but at high intensities, still sufficient tofully absorb the incident light within the focal region.

FIG. 9 illustrates a method that addresses to the problem of 1/r falloffof intensity by ensuring that the light focused on the illuminated line,in the vicinity of the line, will decrease in intensity inverselyproportional to the square of the distance from the line. Light fromlaser 45 successively passes through a series of beam splitters, thefirst two of which are beam splitter 46 and 47, so that the power of thebeam leaving from laser 45 is reflected into sub-beams of equalintensity, directed at a series of lenses 50 to 56. The sub-beamreaching lens 51 is delayed with respect to the sub-beam reaching lens50 because of the added path length from the laser to beam splitter 47compared to the path length to beam splitter 46. Similarly the delayincreases with each beam splitter. The collimated laser light is focusedby lens 50 to point 57, and the light diverging from point 57 isrecollimated by lens 60 to be incident on the lens array, includinglenses 61, 62, 63, 64 plus four unlabeled lenses. Light originallypassing through lens 50, and now refocused by lens 61, passes throughthe surface 2 of ingot 4 to produces an image of point 57 at one spot 68of the illuminated line in ingot 4.

Light passing through lens 51 is similarly focused to a spot on theilluminated line in the ingot 4 adjacent to spot 68. Because of thedelay in the light reaching lens 51 compared to the light reaching lens50, the light reaching spot 68 at the end of the illuminated line neveroverlaps in ingot 4 with the light reaching the point adjacent to spot68. Similarly, for example, light from lens 53, focused to spot 58 andthen refocused to point 69, does overlap in ingot 4 with the lightfocused on endpoint 68 or any of the intervening points. Lens 62 imageslight from lens 50 to a spot on the illuminated line adjacent to thelight from lens 56 imaged by lens 61, and thus the pattern repeats. Inthis way, spots that are separated by six non-illuminated spots areilluminated simultaneously. Close to the illuminated line, the cone oflight focused on any point on the illuminated line does not overlap anyother such cone, so in that range, the intensity falls off inverselywith the square of the distance from the line. At greater distances fromthe line, the instantaneous intensity is never greater than 1/7 of theintensity that would be necessary to create such a line with synchronousillumination. Therefore with two or three-photon absorption, theabsorption at greater distances from the line is no more than 1/49 or1/343, respectively, of the absorption if the line were illuminatedsynchronously.

In case the light for the device of FIG. 9 is generated by laser diodes,then it may be desirable to have a separate laser diode for each of thelenses 50 through 56, with delays between the output pulses of thelasers long enough to avoid overlap near the illuminated line.Alternatively, instead of delay caused by propagation in air, the outputof a single laser could be coupled into a plurality of fibers ofdiffering lengths, to produce the required delays between pulses. In thedevice illustrated in FIG. 9 the number of spots focused on theilluminated line is equal to the number of lenses in the lens arrayincluding lens 50 multiplied the array including lens 61 could includeany number of lenses, so that the number of spots in the illuminatedline could be increased by increasing the number of lenses in eitherarray.

In all the embodiments of the present invention described thus far, thesame laser produces cleavage initiation and extension, however there maybe advantages to using different lasers perform these roles. Forinitiation, a laser in the three-photon absorption band would be used tomore precisely control the location of the stresses. When suchinitiation begins at an edge of the ingot, when the length of the heatedvolume is short, then relatively low peak pulse energy is sufficient.However for cleavage extension, precision of the stress location becomesless important, since the preexisting cleavage serves as a guide tofurther cleavage, but beam power becomes more important because (with anembodiment as in FIG. 6) the heated volume is longer and the morepowerful beams allow faster scanning over the entire wafer. For thisextension phase, a laser in the long wavelength edge of the two-photonabsorption band may be optimal. Thulium doped fiber lasers up to 150 Wcontinuous wave output are now commercially available (IPG PhotonicsCorp. Oxford, Mass.) over the long wavelength cutoff end of the twophoton absorption spectrum of Si, which as discussed above, may be agood choice for the present application. Such a laser, in a mode-lockedor amplified mode-locked configuration, linked by optical fibers to acluster of say 20 separate cleaving workstations, would be probably havesufficient power to drive the entire cluster, at high cleavage extensionvelocities.

For the cleavage initiation phase, lens 11 might be a conventionalSchwarzschild-type reflective objective, imaging an illuminated slit (orline focus of a collimated beam produced by a cylindrical lens) intovolume 13 in the ingot, while the cylindrical lens 11 illustrated inFIGS. 6 and 7 for example, would be used for cleavage extension.

For cleavage initiation, it may be advantageous to image a line of lighton volume 13 at a distance from the axis of lens 11, to put it closer toedge 15 than would be practical if the volume were on the axis of lens11. Such off-axis imaging introduces possible aberrations, which must beaddressed. However it allows a larger effective numerical aperture,which can produce greater resolution than would be possible if theentire lens 11 were moved closer to the edge of the ingot to provideon-axis imaging.

One way to share a laser adapted for cleavage initiation and a secondlaser adapted for cleavage extension, is to use a set-up with just thefirst laser to create multiple cleavage initiation sites in parallellayers on an ingot then move the ingot to a set-up with the secondlaser, where the already initiated cleavage fractures are elongated andthe wafers detached. FIG. 10 shows apparatus, specialized for just thecleavage initiation part of the process, specifically to create a set ofparallel short cleavage fractures on edge 15 of ingot 1, such that eachof these cracks can serve as a starting notch for a successive cleavedwafer. Lens 70, which may be made of silicon or another infraredtransmitting material with a refractive index similar to silicon, has aspherically ground side, and on the side opposite the spherically groundside it has a V-shaped notch, of the same angle as the angle as theintersection of the two sides at edge 15. Therefore, when the notch inlens 70 rests against the ingot 1, the sides of the notch fit tightlyagainst the two sides of the ingot. An immersion fluid of the same indexof refraction as silicon, between the notch of lens 70 and the ingot,may insure good optical coupling of the lens to the interior of theingot. A rectangular slot 71 in lens 70 through the center of thespherically ground surface, has sides normal to edge 15, and the bottom72 of the slot is deep enough so that edge 15 protrudes within the slot71, though this protrusion is not visible in FIG. 10. An extremely sharpdiamond blade 14 (unillustrated) that occupies slot 71 has the samerelationship to edge 15 as in FIG. 7, so that the sharp edge of theblade is perpendicular to edge 15. Lens 70 images a slit shaped sourceof light of an intensity and wavelength able to be absorbed at the focusby three-photon absorption to an illuminated volume (unillustrated) ofsimilar shape to volume 13 of FIG. 7. By changing the position of theslit shaped source imaged on an illuminated volume with successive laserflashes, the volume can be moved progressively farther away from thelens 70, so the resulting cleavage can be not only formed just adjacentto edge 15, but it can be extended a small way into the interior of theingot, to more effectively serve as a seed for further extension of thecleavage across the ingot in later processing. FIG. 11 shows the ingot 1after processing by the apparatus in FIG. 10. Three initiating cleavagesare shown as triangles adjacent to edge 15, though with a more likelywafer thickness, say in the range of 50 μm, there would be 2,000 suchtriangles per 10 cm length of ingot.

The OPA-800C/Spitfire laser commonly has a 1 kHz pulse output frequency,and making the assumption that each pulse can extend the cleavage about5 μm, the growing edge could only advance at about 5 mm/second, whichmay appear not nearly fast enough for a commercially viable wafercleaving operation, especially in view of the high cost of the laser.However it should be remembered that this expensive laser would berequired only at the initiating region of the cleavage, and this regionneed extend only, say, 100 μm beyond the edge 15, so only 20 pulsestogether taking 20 ms would be required to initiate the cleavage perwafer. With an efficient scheme to move from one cleavage site to thenext along the ingot, and to put new ingots in place once the initiationnotches are made in the last ingot, and with around-the-clock operationof the laser, in a year, a single such laser could initiate cleavagesfor 1,576,800,000 wafers.

So that the time for movement of lens 70 from one cleavage site to thenext does not slow down the rate of notch productions, two of thedevices of FIG. 10 could be used in tandem, with a routing switchbetween the laser and the two devices, so 20 pulses from the laser firstwent to one of the devices to produce the first cleavage initiation,then the pulses are immediately routed to the second device while thelens 70 on the first device is moved to the next cleavage site, etc. Ifmore time for translation of the lenses is required, more devices couldbe added, and the laser output routed in sequence through the set ofdevices.

Ingots processed with such initiating cleavages could be transportedaway from a central production facility for final cleavage extension,wafer separation and processing into cells, or the process could be donein a different area of the same production facility.

Once a new wafer is completely cleaved from its parent ingot by themethod of the present invention, the wafer and ingot are still bound byvan der Waals forces, and additionally are compressed by ambient airpressure, or more specifically the viscosity of the air that must enterthe widening crack between the ingot and the new wafer will slow downthe separation process. The adhesion of the new wafer to the parentingot is magnified by the precision of the cleavage process, resultingin two perfect crystal faces in close apposition, and exactly alignedatom by atom. However a variety of methods are available to helpseparate them. Reducing the atmospheric pressure, and/or replacing theair with a less viscous gas like hydrogen, could help alleviate theeffect of atmospheric pressure. A pulse of ultrasonic energy transmittedfrom the end of the ingot opposite the new wafer, could lead tomechanical dislocation and heat formation at the interface layer, andthe thermal gradients induced in the new wafer would produce an archingeffect that could help the separation process. Applying a high voltageto the ingot could induce electrostatic repulsion of the outer layer.The ingot (or more practically, a thick ingot slab) could be spun in acentrifuge with several similar ingot slabs to apply centrifugal forceto the new wafers. Or jets of low density, high velocity gas could bedirected to the notch at the side of the ingot, to apply a separationforce. Various combinations of these and other methods would probably bemore effective than any method by itself.

FIGS. 12A, 12B and 12C illustrate a method of peeling the cleaved waferfrom the rest of the ingot 1. An element 80 with a cylindrical curvature(exaggerated in the drawing) is put in contact with one side 2 of thenewly formed wafer, so the cylindrical surface is tangent to the wafersurface. Some sort of adhesion is provided at this contact point, forexample by vacuum suction, in case the peeling process is done atatmospheric pressure, or perhaps a thermally activated adhesive, or evenby a finger mechanical element that engages with notch 6. It is evenpossible to use water or another liquid at the contact point, whichwould be frozen to promote adhesion. Once adhesion is achieved, element80 is rolled over the surface of the ingot, as shown in FIGS. 12B and12C, such that the newly formed wafer remains in contact with theelement 80.

In case peeling of the new wafer away from the ingot is done in avacuum, then several subsequent processing steps to make solar cellscould be done in the same vacuum environment, to reduce fabricationcosts. These operations include creating selective areas of doping,creating a passivation and antireflection layers and creating metalcontacts. The vacuum chamber could be adapted to cleave multiple wafersfrom the same ingot, to perform initial processing on them, finally tocollect the partially processed wafers in a storage compartment, beforethe vacuum is released, or before the compartment is removed through anairlock.

In such a process, the ingot placed into the vacuum chamber could havemany parallel notches on ingot edge 15, as would be produced by theapparatus in FIG. 10. In that way, the process of cleaving and peelingcould be repeated, each time beginning with the next lower notch. So theseparation between notches can be larger than the wafer thickness, theingot could be ground to have several edges like edge 15 around theperiphery, and successive notches could be on successive edges aroundthe ingot.

In the special case of production of solar cells where all theelectrical connections are from the backside, a transparent stiffeningmember compatible with focusing the light from laser 10 to a focuswithin the ingot, could be applied to face 2 prior to the cleavageprocess. A very thin wafer could thereby be made, with a reduced risk offracture on removal.

In the special case of production of solar cells where all theelectrical connections are from the backside, a transparent stiffeningmember compatible with focusing the light from laser 10 to a focuswithin the ingot, could be applied to face 2 prior to the cleavageprocess. A very thin wafer could thereby be made, with a reduced risk offracture on removal.

For transporting the wafers through a cell processing line, each wafermay be held by a miniaturized vacuum holder, perhaps less than 1 cmthick and the holder would travel through the line along with the wafer.Since the holders would allow only access to one side of the wafer,there could be a reversing operation, where the wafer was lifted fromthe holder, reversed, and then placed back with the other side up. Thevacuum in the holder might be regenerated by connection to a vacuumsystem, at various locations along the production line, or it could evenbe maintained by a extremely miniaturized vacuum pump contained withineach holder.

Another method to reduce fracture of very thin wafers, both duringpeeling from the ingot and during subsequent processing, would be to usea high temperature and low outgassing adhesive tape to tape the wafer toa frame, also made of a material that could withstand the temperaturesand chemical treatments encountered in cell processing. One possiblysuitable tape would be a high temperature and low outgassing polyimidetape with an acrylic adhesive, for example labeling tape sold, forexample, by Polyonics, Inc., Westmoreland, N.H. 03467. The wafer, tapeand frame, would be moved as a unit thorough the various processingsteps. Because the tape would be mask the underlying regions of thewafer from possibly beneficial treatments, perhaps chemical additivescould be added to allow, in conjunction with subsequent heating, anynecessary passivation or doping in such masked regions.

The tape also could help facilitate the peeling of the wafer from theingot. FIG. 13A shows a length of such tape 90, adhering to one edge ofa cleaved wafer still in place on its parent ingot 1. One side of thetape 90 is tapered to a low thickness to reduce local strain on thewafer. The tape could be used in conjunction with a method illustratedin FIG. 12, for example when convex surface of element 80 in FIG. 12A isin contact with one side of the wafer, some means could be provided tobind the cylinder to the tape, so that as element 80 was rolled, itwould pull the tape away from the ingot, thereby freeing the adheringwafer. FIG. 13B shows the frame 91, connected with both the tape 90 andthe wafer. Binding of element 80 to tape 90 could be by a mechanicalfinger (unillustrated) from element 80 that engages one side of frame91, while the frame adheres to tape 90.

FIG. 13C shows in a three-dimensional view and FIG. 14 in a crosssectional view, the completed peeled-away wafer mounted in the frame 91by tape 90 and a second strip of tape 92 on the opposite edge of thewafer. Tape piece 91 could be adhered to the wafer while it was still onthe ingot, but only affixed to the frame after peeling is completed. Asmall tension in the wafer, below the threshold for damage to the wafer,would help keep the wafer taut in the frame, and provide a flat surfacefor processing.

The wafers in their attached frames might be processed in a conventionalsolar cell processing assembly line, provided that contact processeslike screen printing were replaced by non-contact processes such asinkjet printing to reduce stresses on the wafer. However, as illustratedin FIG. 15, in a possibly advantageous alternative type of processing,the frames (e.g., frame 90) would be inserted into “super-frames” (e.g.,super-frame 100) that resemble multipane windows, with the individualframes such as frame 90 analogous to small panes in such windows.

The super-frames might be flexibly linked by coupling elements (e.g.,coupling element 101) to form a chain of super-frames that allows thesuper-frames to lie flat in a row, or to fold in a fan-foldconfiguration. Such a chain could carry the frames and mounted wafersthrough all the wafer processing steps, including the final cell testingand grading operations, without requiring any manipulation of the waferitself. Cell processing based on such chains would have all of the costand throughput benefits of roll to roll processing employed in certainthin film photovoltaic production schemes, while the ability to go intothe compact fan-fold configuration would confer the additionalflexibility to allow a large number of wafers to be maintained in asmall space, for example for a slow vapor diffusion or temperatureannealing operation, without requiring excessive lengths of theproduction line.

The tape might include a conductive strip, which would be connected withconductive strips formed on the wafer, and would allow interconnectionbetween cells without risking fracture of the delicate silicon wafer.Alternatively, if it were not possible to find a tape material able toprovide sufficient adhesion, temperature and chemical resistance at alow enough cost to remain in the finished module, the portion of thewafer adhering to the tape could be severed from the rest of the waferby a cutting laser, and the silicon chemically dissolved so the tapecould be recycled.

It may also be possible to combine the idea of an individualizedminiature vacuum holder with the idea of the fan-folding super-frame,where the vacuum holders were mounted within the individual frame slotsof such a chain of super-frames.

In case the intended cleavage plane contains a hard inclusion, forexample a silicon oxide precipitate, this could prevent the successfulcompletion of the cleavage. In the case of an aborted cleavage due tosuch an inclusion, it might be necessary to remove the ingot from thecleaving apparatus for grinding out the layers containing the inclusion,and such a process would add expense to the process. There are severalpossible solutions to this problem.

The simplest solution is to use a type of silicon ingot that has a verylow concentration of inclusions, for example, float zone grown silicon.This could add cost to the finished cell, but would be more than made upfor by the fact that less silicon is required than in current sawedsingle crystal silicon technologies.

A second possible solution is to saw the initial ingot into, say, 1 cmthick slabs, and to remove the slab from the cleaving apparatus aftereach cleaving. Both the top and bottom surfaces of the slab could becleaved, in preparation for peeling, before the slab is removed, forgreater throughput. With such a system, a fairly high likelihood offailed separation could be tolerated, without substantially adding tothe total cost of the process.

Finally, it may be possible to use confocal reflected infrared lightmicroscopy to inspect each prospective cleavage plane for inclusionsbefore beginning the cleavage process. If an inclusion were found in theplane, then the next prospective plane below that would be so inspected,and if that were found to be free of inclusions, the wafer would becleaved of twice the usual thickness (but still thinner than presentlyused in single crystal solar cells). If both planes were found tocontain inclusions, then the ingot or slab would be removed for grindingand polishing.

Although many microscopic techniques are capable of using reflectedlight to detect scattering particles in a homogeneous medium, theexample used here is a variation of the technique of focal planespecific illumination by means of “disjoint ray envelopes” described inBaer, U.S. Pat. No. 3,547,512, the disclosure of which is incorporatedherein by reference. As shown in FIG. 16, light of a wavelength in thetransparent region of silicon, for example around 1.5 μm, from laser110, which may be a diode laser, is formed into a slit shaped focus byanamorphic beam expander 111 and focused onto slit 112, shown incross-section. Light passing through slit 112 is directed to fullyreflecting mirror 113 so as to pass substantially through one side ofcylindrical lens 114, shown in cross-section. The light directed ontolens 114 is made to pass through a blade-shaped region 115 of thecrystal 116 to converge to line 117, which is the real image of slit112. The light then diverges from line 117 to illuminate anotherblade-shaped region 118. Light reflected from a scattering inclusionparticle lying on line 116 can be focused by cylindrical lens 114 toreach slit 120 (which is conjugate to slit 112 relative to mirror 113 toreach light detector 121. However a scattering particle in illuminatedregion 115 above line 117 or in illuminated region 118 below line 117will be focused by lens 114 to opaque parts of slit aperture 120 socannot reach detector 121. Line 117 is scanned laterally with respect tocrystal 116 (maintaining the geometric interrelationship between slits112 and 120, mirror 113 and lens 114), so that at some time, everyscattering particle in plane 119 can reflect light to detector 121. Noscattering particle above or below plane 119 is ever simultaneously in aregion illuminated by laser 110 and thus able to send light through slit120 to detector 121. The result is that the system can only detectscattering particles in plane 119. The detection of a transientelevation of light by detector 121 during the scan indicates that thereis a scattering particle in plane 119, so another plane should be chosenfor cleavage.

The one-dimensional scanning movement of line 117 with respect to thecrystal is the same as the scanning movement required for cleavage (atthe intended cleavage plane 125) by method of the present invention.Therefore, if the laser apparatus for cleaving and that for inspectionform a rigid assembly that is scanned in one dimension with respect tothe ingot, then both operations can be performed with a single scanmovement. Additionally, the presumptive cleavage plane below plane 119(labeled plane 126 in FIG. 16) could be simultaneously examined duringthis movement, by including duplicates of elements 110, 111, 112 113,114, 120 and 121 adjusted to image this plane 126. In that way, if plane119 is shown unsuitable for cleavage, it will be immediately known ifplane 126 can serve as an alternative.

In the plane inspection method just described, the sides of the ingotmay produce a strong back-reflecting signal which could overwhelm aweaker signal from scattering particles in the plane. This problem couldbe eliminated by using moveable vanes to mask light from passing throughslit 120, specifically in the portions of that slit where the image ofthe edge of the ingot is imaged.

The above apparatus for detecting inclusions will not say where in anyparticular line the inclusion is, only that it is somewhere in the line.However by replacing the detector 121 with a scanned one-dimensionaldetector array, such as sold by Sensors Unlimited, Inc., Princeton,N.J., it will be possible to create an image of the presumptive cleavageplane showing the location of inclusions within the plane. Then it maybe possible to use that information to excise the individual inclusionsby a local milling operation similar to a dentist drilling out a decayedpart of a tooth. The result would be a wafer with one or more very smallholes, which should not substantially reduce photovoltaic conversionefficiency nor increase wafer brittleness, provided the edges of theholes were suitably polished.

FIG. 17 illustrates a variation of the apparatus shown in FIG. 16 toprovide a greater sensitivity to detect inclusions. Pulsed diode laser130 is driven by driver 131 to produce, for example, a 1 ns pulse of1.5.mu.m light every 4 ns. The laser 130 is coupled to output fiber 132,which is split by splitter 133 into a shorter fiber 134 and a longerfiber 135, the difference in the lengths of the fibers causing a delayof the pulse by 2 ns in the longer fiber relative to the shorter fiber.The output of fiber 134 is directed by anamorphic beam expander 136 to aline shaped pattern that is projected on the long slender reflectiveface 137 of (opaque) prism 138 to be directed at mirror 113 (the same asin FIG. 16). Thus the reflective face 137 is analogous to thetransparent opening of slit 112 in the apparatus of FIG. 16. The twin ofeach pulse emitted by fiber 134 is emitted 2 ns later from fiber 135,and directed by anamorphic beam expander 140 to face 141 of prism 138,which then directs the light to mirror 113 and then to the crystal.Because of the timing of the laser pulses, in combination with therelative delay in fibers 134 and 135, every 4 ns a cycle repeats withfirst a 1 ns pulse reflected from face 137 to mirror 113, then a 1 nsdark interval, then a 1 ns pulse reflected from face 141 to mirror 113,then another 1 ns dark interval, etc. This will cause the analog in FIG.17 of line 117 in FIG. 16 to shift back and forth by a small distance,on the order of a micron, on a 4 ns cycle. Reflecting face 150 on prism151 is conjugate to face 137, so that when the laser pulse is reflectedfrom face 137 and focused to a line in the crystal, light emanating fromthat line is focused to face 150. Similarly when light reflected fromface 141 is focused to a line in the crystal, light emanating from thatline is focused to face 152 of prism 151. The light reflected from faces150 and 152 is directed (perhaps via collecting optics) tophotodetectors 153 and 154 respectively. The output of photodetector 153and 154 is sent to combining circuit 155, which also receives input fromlaser driver 131, so that 1) the outputs of photodetectors 153 and 154are gated to zero except for the 1 ns during each 4 ns cycle when theyreceive light from their corresponding illuminated line in the crystal2) the gated output of detector 154 is subtracted from the gated outputof detector 153 to produce a combined output 3) by means of a gaincontrol on the output of one of the detectors, the gated output of thetwo photodetectors are matched as closely as possible, when examining adefect free crystal, so that the combined output (summated over the full4 ns cycle) is zero under those circumstances. When the combined outputfrom circuit 155 is non-zero for a series of consecutive 4 ns cycles,this is a strong indication of a scattering particle in the field of theline illuminated by light reflected from face 137 but not the lineilluminated by light reflected from face 141 or vice versa, in otherwords, that a scattering particle is just entering or exiting the fieldof illumination from light reflected from faces 137 and 141.

The method illustrated in FIGS. 13, 14 and 15 shows an attempt toprovide the advantages of roll to roll processing using individualwafers produced by cleaving a crystal boule. However it is possible togrow crystals of both silicon and sapphire in the form of ribbons. Theseribbons are believed to potentially be much thinner than would befeasible with known processes for drawing ribbons from a melt. Andsilicon in the form of a long ribbon should be far easier to work withthan the arrangement described in FIGS. 13, 14 and 15. By beginning witha ribbon produced by state of the art ribbon production process, thatribbon can be made thinner, for increased flexibility and in some casesphotovoltaic energy conversion efficiency, and of course amultiplication of the area produced. This a method of cleaving ribbonsin cleavage planes parallel to the surface of the ribbon not only atleast doubles the area of ribbon produced, but makes a more valuableproduct. And long lengths of such ribbon can be wound onto rolls forroll to roll processing, that has clear advantages over the method shownin FIGS. 13, 14 and 15.

Crystalline ribbons of silicon and sapphire have been grown by verticalprocesses such as the EGF method and by horizontal processes such astaught for example in U.S. Pat. No. 7,855,087. While the presentinvention can be used with either a vertical or horizontal process, FIG.18 illustrates its use with the horizontal method. A ribbon of siliconor sapphire 160 is drawn from the melt in crucible 161 and pulled in thedirection of arrow 162. Pulsed laser beam 163 focused by cylindricallens 164 induces cleavage between so that ribbon 160 bifurcates into athinner ribbon 165 and 166. The ribbon 165 is wound into a roll. Thethicker ribbon 166, following cleavage, continues in the direction ofarrow 162. A second pulsed laser beam 168, focused by cylindrical lens169, produces a second cleavage, to bifurcate ribbon 166 into ribbons179 and 171 which are wound on respective rolls (unlabeled). The numberof rolls of ribbons produced by this method is arbitrary, so that if thewidth of ribbon 160 were 400 μm, it would be possible to create 16 rollsof ribbon, each ribbon with a 20 μm thickness. In this way it may bepossible to create thin ribbons of crystal, adapted for roll to rollprocessing, extremely economically. It should be said that theinitiation of the cleavage could be done as described earlier in thisspecification, starting with a notch. But the initiation process, oncebegan, will not have to be repeated on any ribbon, until the rolls havereached their maximum lengths.

FIG. 19 shows how the ribbons cleaved from the ribbon originally pulledfrom the melt in a ribbon process, can be laminated together with theribbon cleaved from another melt. The numerals for the elements for theleft hand crucible and ribbon are the same as in FIG. 18, while thenumerals for the right hand crucible and ribbon are shown by primes. Theprocess of bonding the two ribbons could be facilitated by pressure,heat, ultrasonic energy, laser irradiation etc. As an example of such aprocess to facilitate such bonding, the two rollers 172 and 173 areshown for applying pressure, perhaps in conjunction with some otherenergy input.

The two ribbons that are laminated in the apparatus shown in FIG. 19could be of the same material, eg., silicon or sapphire, but havedifferent crystal orientations, so that, like plywood, that the laminateis stronger than its component layers. Also the laminate can be from twodifferent forms of the same material, for example p-doped and n-dopedsilicon, so that the lamination process can create heterojunctionphotovoltaic cell. It is also possible for the two ribbons laminatedtogether to be of different substances, for example one of silicon andthe other of sapphire. Such a laminate could provide a cell with thedurability of sapphire combined with the photovoltaic efficiency ofsilicon. When sufficiently thin, both silicon and sapphire have a lowcost per unit area, so if the production costs could be low enough, thisparticular laminate might be competitive with conventional buildingskins that do not offer photovoltaic conversion.

The particular schematic diagram shown in FIG. 19 does not appear tooffer much physical space for any additions to either cleaved ribbonfrom the time it is produced to the time it is laminated. However bysimply rearranging the geometry, there would be space for a wide rangeof modifications, such as adding conductive layers. A particularlyuseful modification might be the insertion of an intermediate layerbetween the two laminated cleaved ribbons 165 and 165′ in FIG. 19. Suchan intermediate layer could improve the strength per weight per unitarea, by the use of a light weight plastic between two outer layers ofsapphire. The intermediate layer could also improve adhesion between theouter layers.

The cleaving method of the present invention has been described withreference to silicon and sapphire, however other crystalline materialssuch as germanium, gallium arsenide, silicon carbide and diamond alsohave sharp transitions from transparency to opacity at a particularcritical wavelength, so could be used in a similar wafer-cleavingscheme. In general the requirement for the present technique is that forat least one wavelength, the material is relatively transparent for lowintensities, where single photon absorption predominates, but isrelatively opaque at high intensities where two-photon or three-photonabsorption is significant.

One of the goals of this technique was to provide a technique that couldbe carried out a room temperature and in a normal atmosphere or a vacuumenvironment. However it may be that under some circumstances, otherconditions produce better results.

The present invention has been described here as focusing light on thedesired cleavage plane, using two-photon or three-photon absorption ofthat light to produce selective heating of the focal volume, howeverother types of radiation that have the properties of allowing formationof a local heated region in a crystal, without substantial heating ofthe overlying layers and without leaving any permanent damage to thecrystal, might substitute for light and multi-photon absorption.

The volume of the specimen onto which the light from laser 10 wasfocused in the embodiment described in this specification was a singleapproximate cylinder with an axis parallel to the growing fracture edge,and elongated in the dimension of the ingot axis to have a generallyoval cross section. However alternatively shaped illuminated volumes maybe advantageous in some applications of the present invention, forexample a disjoint volume including two such distorted cylinders ofillumination, one ahead of the cleavage growth front and the otherbehind the front, so that the two regions together provide a tensilestress field around the growing fracture, that causes the forces to moreprecisely be normal to the plane of the fracture, or are more tolerantto errors in the depth of focusing of the illuminated volume(s) inrelationship to the plane of the desired cleavage.

This description has described the ingot of silicon as oriented in the(111) axis, however for some uses, some other orientation of the wafersmay be desirable, and substitute for the (111) orientation. It may evenbe possible to apply this technique in amorphous solids such as fusedsilica with no preferred cleavage planes; since the tensile forces areapplied so precisely at the growing fracture front, a planar fracturemay be possible in the absence of a crystal cleavage plane. In the caseof fused silica, for example, an intense pulse at 0.3 micron wavelengthcould induce two-photon absorption or a pulse at 0.45 microns couldinduce three-photon absorption, and hence rapid local heating requiredto implement the present invention.

This method of wafer production has been presented herein as analternative to the method of layer weakening by hydrogen ionimplantation, however it might also be used in conjunction with thistechnique, allowing a smaller does of ions, and a shorter annealingtime, for faster throughput and lower cost.

In the case of sapphire, the crystal is transparent at wavelengths aboveabout 250 nm. Therefore it might be possible to use the widely availabletitanium-sapphire mode-locked laser tuned to 800 nm, and frequencytripled to 266 nm by SBO crystals (Eskma Optics, Vilnius, Lithuania).Such light then can be absorbed by two photon absorption in the sapphirecrystal. Light of an appropriately longer wavelength will be too longfor two photon absorption, but can be efficiently absorbed in thesapphire crystal by three-photon absorption.

This specification described the cleavage being triggered at a notch.However other additions or subtractions to crystal that will initiate afracture at the chosen cleavage plane when exposed to tensile stresscould substitute for such a notch, and may include volumes where thecrystal structure was weakened due to exposure to focused beams ofvarious kinds of radiation.

Thus the scope of the invention should not be limited to the embodimentsdescribed but rather to the following claims.

It is claimed: 1) A method for cleaving a crystal at a chosen plane,said crystal having a surface parallel to said plane, includingproviding a beam that passes trough said surface in a direction normalto said plane to converge on a focal area in said chosen plan, timingsaid beam so it heats said focal region rapidly enough to induce tensilestresses in said crystal sufficient to induce fracture at said plane,and moving the location of said heated region through said crystal, tospread said fracture over said chosen plane, thereby cleaving saidcrystal at said chosen plan. 2) The method of claim 1 wherein saidcrystal is silicon. 3) The method of claim 1 wherein the step of heatinga region in the interior of said crystal includes the step of providingradiation for which the crystal is transparent when said radiation has alow intensity, but which the crystal absorbs when said radiation is at ahigh intensity. 4) The method of claim 3 wherein said crystal absorbssaid radiation by the process of two-photon or three-photon absorptionwhen such radiation is provided at a high intensity. 5) The method ofclaim 6 wherein said radiation has a wavelength between 1 and 3.5microns. 6) The method of claim 5 wherein the source of said radiationcomprises a mode-locked laser. 7) The method of claim 1, wherein thestep of heating a region comprises providing a cylindrical lens. 8) Themethod of claim 1, wherein the step of heating a region includes thestep of scanning the focused beam of a laser along a line within saidregion. 9) The method of claim 1, wherein the step of heating a regionincludes providing a laser adapted to producing pulses of light,directing each pulse to at least a first spot and a second spot withinsaid region, and delaying the conduction paths between said laser andsaid first spot and between said laser and said second spot by differentdelays, so that each pulse arrives at the said first spot at a differenttime than the pulse arrives at said second spot. 10) The method of claim1, including the steps of providing a stiffening member and attachingsaid stiffening member to said crystal. 11) The method of claim 1,including a step of reducing the reducing adhesion of the parts of saidcrystal on the two sides of said cleavage. 12) The method of claim 11wherein said step for reducing adhesion includes providing gas betweensaid parts. 13) The method of claim 2 wherein said crystal is growingwhile said cleaving is taking place, and said wafer is in the form of acontinuous ribbon. 14) A wafer produced by the method of claim
 1. 15)Apparatus for cleaving a crystal at a chosen plane including on saidcrystal a structure which when exposed to tensile stress can initiate afracture at said chosen plane, an optical element for directing lightthrough a face on said crystal parallel to said chosen plane to a regionin the interior of said crystal rapidly enough so that the resultingtensile stress at said structure is sufficient to produce fracture atsaid plane, and a scanning component for moving the location of saidheated region through said crystal, to spread said fracture over saidchosen plane. 16) Wafers produced by the apparatus of claim
 15. 17) Theapparatus of claim 15 where said crystal is a semiconductor in a groupincluding but not limited to silicon, germanium, and gallium arsenide.18) The apparatus of claim 15 where said crystal is sapphire or diamond.19) A method for cleaving a material at a chosen plane including:providing on one surface of said material a structure which when exposedto tensile stress can initiate a fracture at said chosen plane, heatinga local region in the interior of said material, rapidly enough so thattemperature gradients can form sufficient to create tensile stresses atsaid fracture, normal to said fracture and moving the location of saidlocal region through said material, to promote the elongation of saidfracture over said chosen plane. 20) The method of claim 19 wherein saidmaterial is an amorphous solid in the class including glass and fusedsilica.